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Showing 71 to 74 of 74 (0.041 seconds)| 71 |
Dopustiva singularna rešenja sistema gasne dinamike sa nepozitivnim pritiskom / Admissible singular solutions to gas dynamics systems with non-positive pressureRužičić Sanja 23 June 2020 (has links)
<p>Karakteristika hiperboličnih sistema zakona odrržanja je da čak i u slučaju glatkog po-četnog uslova rešenja uglavnom razvijaju prekide u konačnom vremena. Zbog toga se posmatraju slaba rešenja koja dati sistem zadovoljavaju u distributivnom smislu i mogu biti čak i neograničena što se ispoljava kroz pojavu Dirakove delta funkcije u rešenju. U ovoj disertaciji se akcenat stavlja na analizu protoka stišljivog neviskoznog fluida koji ne menja pravac prilikom kretanja. Protok je opisan Ojlerovim sistemom iz gasne dinamike koji se sastoji iz zakona održanja mase, količine kretanja i energije, dok su karakteristike fluida određene konstitutivnim relacijama. U slučaju izentropskog ili izotermnog protoka sistem se svodi na zakone održanja mase i količine kretanja. Glatka rešenja takvog sistema automatski zadovoljavaju zakon održanja energije, dok prelaskom na slabu formulaciju dolazi do gubitka energije. Za predstavnike sistema gasne dinamike sa nepozitivnim pritiskom su uzeti sistem gasne dinamike bez pritiska i model za Čapliginov gas i njegova uopštenja. Data su rešenja Rimanovih problema za te sisteme koja se mogu predstaviti kao kombinacija klasičnih elementarnih talasa i senka talasa koji aproksimiraju rešenja u obliku delta udarnih talasa i koji omogućavaju rešavanje početnog problema koji u početnom uslovu sadrži delta funkciju. Na primeru modela za uopšten Čapliginov gas dokazano je da uslov prekompresivnosti nije jači od entropijskog uslova, što je prvi takav rezultat u literaturi. Dalje su korišćena rešenja Rimanovih problema, kao i problema singularne interakcije i dat je algoritam za konstrukciju globalnog dopustivog približnog rešenja početnog problema za sistem gasne dinamike bez pritiska. Algoritam je univerzalan i ideja se može proširiti na veliki broj sistema zakona održanja i veliki broj početnih uslova. Diskutovane su promene energije u približnom rešenju i posle interakcija. Dobijeno približno rešenje slabo konvergira u prostoru Radonovih mera sa predznakom.</p> / <p> </p><p class="MsoNormal">A solutions to hyperbolic conservation laws systems starting out as smooth often develop singularities in a finite time. As a consequence, we are forced to look for weak solutions that satisfy the system in distributional sense. Those solutions are often unbounded, which is expressed through the appearance of Dirac delta function. In this theses we study a one-dimensional, compressible and inviscid flow of a fluid. The process is described by compressible Euler gas dynamics system which consists of conservation laws of mass, linear momentum and energy, while the characteristics of the fluid are described using constitutive relations. In the case of isentropic or isothermal flow the system reduces to conservation laws of mass and linear momentum. The energy is conserved for smooth solutions to such systems, but while passing to the weak formulation the energy is being dissipated. As representatives, we consider pressureless gas dynamics system, as well as Chaplygin gas model and its generalizations. We give the solutions to Riemann problems which can be represented as a combinations of classical elementary waves and shadow waves that approximate the solutions in the form of delta shock and allow as to solve the problems with initial data containing delta function. We use generalized Chaplygin gas model as demonstration of the fact that overcompressibility condition is not stronger that entropy condition, which is the first result of that kind in the literature. Further, we use solutions to the Riemann problems, as well as singular interaction problems to give the algorithm for construction of global admissible approximate solution to the pressureless gas dynamics initial value problem. The algorithm is universal and idea can be applied to large number of conservation laws systems and large number of initial data. We discuss energy changes in approximate solution and after the interactions. The constructed approximate solution converges in the space of signed Radon measures.</p><p><!--[if gte mso 9]><xml> <w:WordDocument> <w:View>Normal</w:View> <w:Zoom>0</w:Zoom> <w:TrackMoves/> <w:TrackFormatting/> <w:PunctuationKerning/> <w:ValidateAgainstSchemas/> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:DoNotPromoteQF/> <w:LidThemeOther>EN-US</w:LidThemeOther> <w:LidThemeAsian>X-NONE</w:LidThemeAsian> <w:LidThemeComplexScript>X-NONE</w:LidThemeComplexScript> <w:Compatibility> <w:BreakWrappedTables/> <w:SnapToGridInCell/> <w:WrapTextWithPunct/> <w:UseAsianBreakRules/> <w:DontGrowAutofit/> <w:SplitPgBreakAndParaMark/> <w:DontVertAlignCellWithSp/> <w:DontBreakConstrainedForcedTables/> <w:DontVertAlignInTxbx/> <w:Word11KerningPairs/> <w:CachedColBalance/> </w:Compatibility> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> <m:mathPr> <m:mathFont 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Distribucija i dinamika populacija najznačajnijih grupa polinatora u agroekosistemima Vojvodine / Distribution and dynamics of populations of the most important groups pollinators in the agro-ecosystems of VojvodinaMudri Stojnić Sonja 29 August 2018 (has links)
<p>U radu je prikazana distribucija, dinamika i diverzitet insekata oprašivača iz reda Hymenoptera - Apiformes (Anthophila) i Diptera Syrphidae) na stepskim fragmentima i na suncokretu u agroekosistema Vojvodine. U cilju uvida u strukturu predela injenog uticaja na sastav i brojnost polinatora, kartirani su tipovistaništa oko svakog stepskog fragmenta. Na osnovu podataka dobijenih kartiranjem, odabrano je sedam stepskih fragmenata kojiu svom okruženju imaju visok udeo suncokreta kao masovnocvetajuće kulture i sedam stepskih fragmenata koji su bez ili saniskim udelom suncokreta. Iz reda Hymenoptera – Apoideazabeleženo je šest familija: Andrenidae, Apidae, Colletidae,Halictidae, Melittidae i Megachilidae, 114 vrsta, a iz reda Diptera(Syrphidae), registrovano je ukupno 11 vrsta. Predstavnici familija Andrenidae, Apidae i Halictidae su distribuirani na svim lokalitetima,predstavnici familije Megachilidae su distribuirani na 15 od 16lokaliteta, a najmanje su zastupljene jedinke familija Colletidae i<br />Melittidae, distribuirane na pet lokaliteta. Polinatori reda Diptera familije Syrphidae su distribuirani na svim lokalitetima. Rezultati Kruskal-Volisovog H testa ukuzuju da je tokom sve tri sezone (2011.,2012., 2013.) na stepskim fragmentima najviše bilo zastupljeno vrsta solitarnih pčela, zatim vrsta osolikih muva, a najmanje vrsta bumbara. Istim testom je dobijano da je tokom sve tri sezone na stepskim fragmentima, registrovano najviše jedinki osolikih muva,<br />zatim medonosne pčele, solitarne pčele, a najmanje jedinki bumbara. Fridmanovim testom su utvrđene razlike u brojnosti (dinamici) polinatora kroz sezone, uočen je porast broja jedinki medonosne pčele i opadanje broja jedinki solitarnih pčela.Rezultati dobijeni Man-Vitnijevim U-testom pokazuju da je na<br />stepskim fragmentima koji imaju niži udeo suncokreta u predelu zastupljeno više jedinki i vrsta bumbara. Istim testom je dobijen rezultat da je na stepskim fragmentima sa visokim udelom suncokreta ima više jedinki medonosne pčele. Vilkoksonovim testom sume rangova je pokazano da su jedinke i vrste bumbara zastupljenije na stepskim fragmentima nakon cvetanja suncokreta, za vreme cvetanja suncokreta na stepskim fragmentima je registrovano više jedinki <em>Apis mellifera</em>, osolikih muva i solitarnih pčela. Modeli regresionih analiza linearnih mešovitih modela su pokazali da se sa porastom udela suncokreta u predelu smanjuje broj jedinki divljih pčela i jedinki i vrsta bumbara. Sa porastom udela polu-prirodnih staništa u predelu i većom cvetnom pokrovnosti, povećava se udeo jedinki i vrsta osolikih muva.</p> / <p>This paper shows distribution, dynamic and pollinator diversity Hymenoptera - Apiformes (Anthophila) and Diptera (Syrphidae) in semi-natural habitats and in sunflower crops in Vojvodina agroecosystems. Around each of 16 selected steppe fragments, habitat types were mapped to test how do landscape structure affects pollinator diversity and abundance in semi natural habitats and in sunflower crops. Based on the results obtained by mapping, seven study sites with high % of sunflower like mass flowering crops, and eight study sites with no or low % of mass flowering crops are selected. In total, there were 114 species from 6 families from order Hymenoptera-Apiformes: Andrenidae, Apidae, Colletidae, Halictidae, Melittidae and Megachilidae, and 11 species from order Diptera (Syrphidae). Insects from families: Andrenidae, Apidae, Colletidae and Halictidae were distributed on all study sites, while insects from family Megachilidae were distributed almost on all study sites (15 sites). At least only on five study sites were distributed insects from family: Colletidae and Melittidae. Hoverflies were distributed on all study sites. Kruskal-Wallis H test shows that an all three seasons (2011., 2012., 2013.) in semi natural habitats wild bees species were most abundant, followed by hoverfly species, and bumblebee species at the end. Same test shows that in all three seasons in semi natural habitats individuals of hoverflies were more abundant than individuals of honey bees, wild bees and individuals of bumblebees, which were least abundant. Friedman test shows differences in densities of pollinator through the seasons, and these results shows increasing in Apis mellifera densities and decline of wild bees densities through seasons. Man-Whitney U-test shows that there were more species and individuals of bumble bees in semi-natural habitats which landscapes are without or low % of sunflower. Same test shows that there were more individuals of honey bees in semi-natural habitats which landscapes have high % of sunflower. Wilcoxon signed-rank test shows that in semi-natural habitats species and individuals of bumblebees were more abundant after blooming sunflower, while species and individuals of wild bees as well as individuals of hoverflies and <em>Apis mellifera</em> were more abundant during blooming sunflower. Linear mixed-effect model shows that with increase of % of sunflower in landscape number of individuals of wild bees and species and individuals of bumblebees decreasing, and individuals of hoverflies increasing. With an increase of % of semi natural habitats and increase of flower cover, abundance and species of hoverflies<br />increases.</p>
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Automobilio svyravimai ekstremalaus stabdymo metu / Oscillations of the vehicle in emergency brakingPečeliūnas, Robertas 24 February 2005 (has links)
The suspension of the motor vehicle is one of the most important elements of the flexible mounted and inflexible mounted masses of the vehicle bodywork, and most attention is paid to its exploration and improvement. Analysis of models of equivalent oscillation systems of motor vehicles testifies that the evaluation of motor vehicle oscillations and modelling of its motion modes is still very topical and requires further research. Suspension models of motor vehicles offered in publications regard only the influence of road irregularities, and the modernisation of these models is directed towards the improvement of passengers’ comfort. However not much research has been done on the influence of oscillations of flexible mounted and inflexible mounted masses of the bodyworks of vehicles in the process of braking; also there is not much investigation of the post-accident identification of the vehicle’s movement mode corresponding to the deformations of the suspension and the longitudinal pitch of the bodywork. Research of oscillations in the conditions of emergency braking is primarily important for the work in two practical directions:
1) improvement of calculation methods of motor vehicle’s response to external impact in the conditions of real operation;
2) further improvement of research methods on the road, and analysis of fait accompli road accidents related to oscillations in the emergency braking.
Methodology defining the oscillations occurring in the braking may be applied... [to full text]
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Automobilio svyravimai ekstremalaus stabdymo metu / Oscillations of the vehicle in emergency brakingPečeliūnas, Robertas 25 February 2005 (has links)
The suspension of the motor vehicle is one of the most important elements of the flexible mounted and inflexible mounted masses of the vehicle bodywork, and most attention is paid to its exploration and improvement. Analysis of models of equivalent oscillation systems of motor vehicles testifies that the evaluation of motor vehicle oscillations and modelling of its motion modes is still very topical and requires further research. Suspension models of motor vehicles offered in publications regard only the influence of road irregularities, and the modernisation of these models is directed towards the improvement of passengers’ comfort. However not much research has been done on the influence of oscillations of flexible mounted and inflexible mounted masses of the bodyworks of vehicles in the process of braking; also there is not much investigation of the post-accident identification of the vehicle’s movement mode corresponding to the deformations of the suspension and the longitudinal pitch of the bodywork. Research of oscillations in the conditions of emergency braking is primarily important for the work in two practical directions:
1) improvement of calculation methods of motor vehicle’s response to external impact in the conditions of real operation;
2) further improvement of research methods on the road, and analysis of fait accompli road accidents related to oscillations in the emergency braking.
Methodology defining the oscillations occurring in the braking may be applied... [to full text]
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